While there exists many types of filters for removing particulate matter from an influent, such filters are generally classified as the type having a bonded or nonbonded media. A bonded media filter includes a removable cartridge element constructed of a fibrous woven or nonwoven material. The material can be selected with a given porosity so that particulate matter of a given size can be removed from the influent. When the bonded cartridge filter element has a sufficient accumulation of filtered matter thereon, it is simply removed and cleaned, or replaced. The cartridge type filters are not easily backwashed. However, many cartridge-type filters are of the radial-flow type, whereby a maximum surface area is provided for filtering, thereby allowing a reduced resistance to the flow of the influent.
Another family of filters contains a nonbonded media, such as sand, glass beads, diatomaceous earth and other granules or particles through which the influent flows. The nonbonded media is generally of a granular type of material, circular, rounded or irregular in shape so that the spacing between the particles is effective to filter the particulate matter. The advantage of utilizing a nonbonded media filter is that it can be backwashed to regenerate the media. Backwashing can include the fluidizing of the media which allows the fluid to dislodge the entrapped contaminants from both the interstices between the grains of the media, as well as from the surface of each grain itself. The primary disadvantage of such type of filter is the size requirements and costs, as well as filter inefficiencies, in that they have little surface area of the filter exposed to the incoming flow, and thus are forced to utilize larger media grains and higher flow rates per unit area exposed to the incoming flow. In other words, the development of a radial-flow, nonbonded media filter that can be regenerated by backwashing is not a simple task.
In U.S. Pat. No. 3,415,382, by Martin, there is disclosed a radial-flow filter utilizing glass beads as the nonbonded media. While such filter is effective for its intended purpose, it utilizes a rather large-size bead media and can not be regenerated without disassembly.
Radial-flow filters have a broad range of applications in the manufacturing or process industries which require the removal of impurities or solids from an influent. A generalized diagram of a basic radial-flow filter 10 is shown in FIG. 1. The filter consists of two concentric perforated pipes 12 and 14 and a porous filter media 16 filling the annular space 20 between the two pipes, all housed within a filter case 18. The porous media 16 is composed of tiny glass spheres which are of uniform size for a particular filter but can range widely in size for different filters. The spheres can be submicron sized, micron sized or as large as coarse sand, and completely fill the compartment 20 between the perforated pipes 12 and 14. The perforations in the pipes are circular, of uniform size and arrayed in a uniform pattern, but it can be of other arrangements. The concentric-pipes-porous-medium assemblage is encased so that fluid completely surrounds the assemblage during filtration. Filtration takes place along the entire axial length of the filter 10 as the fluid flows radially into the porous media 16 through the perforations in the outer pipe 12, and exits the porous media 16 through the inner perforated pipe 14. The impurities are trapped as the fluid traverses the porous media 16.
The porous media 16 must be cleaned by backwashing after one or more filtration cycles. Backwashing consists of surges of clean fluid that flows radially outwardly from the inner pipe 14, into the porous media 16 and out through the outer perforated pipe 12. The direction of flow is basically opposite to that which takes place during filtration. FIG. 2 shows the filter 10 during a conventional backwash cycle. The relatively high fluid velocities and surges that are generated around the glass spheres dislodge and flush out the accumulated impurities. The impurities are sufficiently small to pass through the spaces between the glass spheres that comprise the porous media 16. However, not all of the impurities are able to be dislodged as a gum residue and particles gradually build up in the porous media 16. Therefore, after a number of filtration backwashing cycles, the filter 10 must be disassembled to replace or recondition the porous media 16.
From the foregoing, it can be seen that a need exists for a radial-flow filter of the type employing a nonbonded media, and constructed so that backwashing capabilities are afforded. Another need exists for a nonbonded media filter constructed such that during the backwashing cycle, the porous media is completely regenerated, thereby eliminating the need to periodically disassemble the filter and completely clean the same or replace the porous media. Another need exists for a nonbonded media filter of the type that can be backwashed, and where the backwashing pressures need not be excessive. Another need exists for a filter of the type where the end of a backwash operation results in a high restriction to the flow of the backwash liquid, thereby increasing the pressure of the liquid and signaling that the backwash operation is complete.